Method for integrating sodium based sulfite and sulfate pulping processes



March 7, 1961 W. A. BIGGS, JR., ETAL METHOD FOR INTEGRATING SODIUM BASED SULFITE AND SULFATE PULPING PROCESSES Filed May 19, 1955 N607'R4L 50!. W75 am. Hrs sne/vr SULFATE I How SULFATE mu out,

courc/vmnn-o COIVCfi/Y TRATE D SPENT LIQUOR SPENT LIQUOR I ACID/F150 COIYCEIYTIMTED SPENT LIQUOR SOLVENT ACE TIC ACID FORM/C ACID WAXE 3 TAN/WW8 RES/NS 077/6 EXTRACT/V55 l nrn/m r5 MIXED SULFATE SPENT LIQUOR 6 RAFFl/VATE CONCENTRATED MIXED \SULFATtI SPENT LIQUOR 6 fiflFF/IYATE SMCLT I m uouon' wmrz 1.10000 IN VEN TORS WILLIAM A BIGGS, J (IL/U6 D. 808587150 ATTOR/YS Y3 Patented Mar. 7, 1961 ice METHOD FOR INTEGRATING somUM ar isan SULFITE AND SULFATE PULPING Pnocnssns William A. Biggs, Jr., and Julius D. Robertson, Hartsville, S.C., assignors to Sonoco Products Company, a corporation of South Carolina Filed May 19, 1955, Ser. No. 509,694

4 Claims. (Cl. 162-33) This invention relates to a process for integrating sodium based neutral sulfite pulp operation with a kraft, sulfate, pulp operation, and more particularly to an improvement in such a process by which the desired sulfidity in the final sulfate white liquor is obtained without the usual necessity of burning of sulfur and adding of salt cake in the smelting furnace. Instead of that latter expensive procedure, the process of the present invention obtains the desired sulfidity, with concomitant improvements in yield and quality of pulp, by the use of a previously processed sodium based neutral sulfite spent liquor, and especially by the effective removal from such liquor of the acetic and formic acid constituents. If those acids were carried over into the sulfate white liquor they would produce non-sulfur bearing sodium compounds and thereby contribute to an undesirably low sulfidity in the White liquor.

In accordance with the present invention, the preliminarytreatment of the neutral sulfite spent liquor comprises concentrating the spent liquor to about 25% to 60% concentration, followed by acidification with sulfuric acid which liberates the acetic and formic acids and enables their removal by proper solvent extraction such as, for example, treatment with methyl ethyl ke tone or other suitable organic solvent. At the same time, the sulfuric acid converts the non-sulfur bearing sodium compounds to sulfur bearing sodium compounds, e.g., conversion of the sodium formate, sodium acetate and sodium carbonate, to sodium sulfate, in situ. In this manner the sulfidity of the neutral sulfite spent liquor, with, acetic and formic acids removed, is effectively increased from a usual or normal sulfidity of about 40% to 100% up to a sulfidity of about 120% to 150%. Accordingly, when this liquor is integrated or combined with the sulfate spent liquor and the mixture subsequently smelted and causticized, the sulfidity of the resulting white liquor is. increased up to about 25% to 35%. This desirable high sulfidity in the white liquor is thus obtained without buring any sulfur or adding any salt cake to the liquorin the furnace as is commonly required. Moreover, the in situ formation of sodium sulfate in the sulfite spent liquor yields higher sulfidities than can be obtained by the addition of any equivalent amounts of salt cake as make-up.

An important feature of this improved process is not only the liberation but actual removal of the acetic and formic acids from the neutral sulfite spent liquor before it is mixed with the sulfate spent liquor. If these acids were permitted to remain in the neutral sulfite liquor they would react'with the sodium hydrogen sulfide of the sulfate spent. liquor to. form hydrogen sulfide gas, which would result in loss .of sulfur needed for maintaining the high sulfidity.

From the standpoint of economic feasibility in operation and maximum strength and yield of pulp, it has been recognized heretofore that the preferred process for pulping hardwoods is the sodium based neutral sulfite process, and that the preferred process for pulping soft woods is the sulfate process. The demands of the paper industry have rapidlydepleted the supply of soft woods, while hardwoods are available in relative abundance. The situation therefore favors I an increase in hardwood pulp production. Heretofore, the sodium based neutral sulfite mill has faced certain difficulties and disadvantages. For example, it is difficult to recover any of the chemical values of the spent liquor for re-use in the process, so much so, in fact, that most such mills not combined with a sulfate mill, make no attempt to recover them. Aside from the waste, the disposal of all or part of the efiiuent, which contains organic sub stances of a high bio-chemical oxygen demand, creates a serious problem in public relation through stream pollution. The desirability of a system which could utilize the otherwise wasted chemical and heat values and eliminate the disposal problem is therefore obvious.

It was partially to solve these difficulties that combination of the sodium based neutral sulfite process with the sulfate process has been practiced heretofore to some extent. conventionally, according to present practice, the pulp mill black liquor eflluent from the sodium based neutral sulfite process, hereinafter called sulfite spent liquor, or a portion thereof, is added to the black liquor from the sulfate process. Salt cake is added to the system to supply the remaining sodium and sulfur values that cannot be supplied by the sulfite spent liquor. The mixture is burned in a recovery furnace and the resulting smelt is dissolved to produce sulfate green liquor. The green liquor is then causticized with lime to produce the white cooking liquor for the sulfate process.

The principal disadvantages of the foregoing conventional system are the following:

, The sulfate cooking process is carried out in a white liquor having a lower sulfidity than is desirable. Sulfidity, as used herein, is defined as the percent satisfaction of the sulfur demand of the sodium, based on one gram atom of sulfur reacting with two gram atoms of sodium.- This sulfidity factor is of vital importance in the sulfate process, since the strength and yield of the product depend upon the alkali-sulphur balance. The desirable range of sulfidity in sulfate white liquor, from the standpoint of maximumyield and maximum strength, is generally accepted as from 20% to 35%. In the sulfate process, there is an inevitable loss of sodium in the sulfate black liquor upon pulp digestion, smelting and causticizing; there is also an inevitable loss of sulfur,

much higher percentagewise than the loss of sodium] These losses must be made up; conventionally, sodium and sulfur losses are made up by adding salt cake, which has a theoretical sulfidity of 100%, .prior to smelting. However, the loss of sulfur in the smelting operation is again higher percentagewise than the loss of sodium.

The difficulty of maintainng sulfidity is due to the disproportionately high lossv of sulfur as compared with sodium. The .best method heretofore practiced to maintain the necessary sulfidity is the addition of elemental sulfur as well as salt cake to the sulfate black liquor prior to burning; here, much of the sulfur is lost in the flue gases as S0 pensive and inefiicient. As will be pointed out more The method is, therefore, ex

fully below, the sulfate white liquor can be maintained at a higher sulfidity by the process of our invention, Without the addition of salt cake and elemental sulfur as make-up, than can be economically maintained by conventional practice, using salt cake and elemental sulfur as make-up.

The addition of concentrated sulfite spent liquor to the sulfate black liquor prior to burning, as presently practiced, results in lower sulfidity than can be obtained by using salt cake alone as make-up chemical. The sodium acetate and sodium formate present therein burn to sodium carbonate, requiring the addition of so much sulfurbearing make-up chemical to off-set the resulting sulfidity-lowering that the ratio of chemical to wood is in excess of the desirable ratio for efficient operation of a sulfate mill. By the process of our invention this problem is eliminated.

Common practice in combined sulfite and sulfate systems is to minimize the sulfidity loss, resulting from sodium carbonate formation and from sulfur loss, by restricting the amount of sulfite spent liquor to be combined with the sulfate spent liquor, thus restricting the tonnage of sulfite pulp produced, to the extent that ordinarily only one ton of sulfite pulp is produced for every four or five tons of sulfate pulp. The remainder of the chemical demand must still be met by the addition of salt cake and sulfur. According to our invention, this ratio can be increased so that one ton or more of sulfite pulp may be produced for every ton of sulfate pulp.

Under present conditions of sulfate and sodium based sulfite operation, there is no practicable combination by which the chemical demand of a sulfate mill can be supplied by waste chemical from a sodium based sulfite mill without the addition of further make-up chemicals during the smelting operation.

The accompanying drawing illustrates the process of the present invention in flow diagram form, the ingredients involved being labeled in this fiow diagram.

In accordance with a typical example of the process of our invention, the sodium based sulfite cooking liquor, whose active ingredients are sodium sulfite and sodium bicarbonate, is cooked with wood chips and steam under pressure, in the conventional manner. The spent liquor, containing principally sodium lignin sulfonate, sodium acetate, sodium formate, sodium bicarbonate. waxes, tannins, resins, and sugars, is concentrated. We have found that the best practicable concentration is in the range of about 40% to 45% solids content, although concentrations ranging from 25% to 60% solids content may be used. To the concentrated liquor is added a sufficient quantity of sulfuric acid to convert all sodium acetate, sodium formate, and sodium bicarbonate to sodium sulfate, and to liberate formic and acetic acids. able organic solvent is added and the concentrated acidified liquor is extracted, to remove the acetic acid, formic acid, waxes, tannins, resins, and other extractives. The steps of concentrating, acidifying, and extracting are described and disclosed in prior co-pending application Serial No. 369,742, filed July 22, 1953, now Patent No. 2,714,118. The products obtained on rectifying the extract can be processed and sold in sufiicient quantity to pay for part or all of the raw chemicals used in the entire integrated system. The raffinate from the extraction is used to supply the entire sodium and sulfur demand of the sulfate process as pointed out below.

The extracting solvent used may be any solvent or solvent mixture which insofar as is practicably possible prevents emulsion formation in the concentrated acidified sulfite spent liquor and at the same time has a high partition coefficient for the extraction of acetic and formic acids from the liquor; and whose water binary azeotrope is such that the solvent and the water may be separated from the acids by distillation. Depending on the choice of solvents, waxes, tannins, and resins may also be extracted simultaneously. As disclosed in the above-men- A suittioned prior co-pending application Serial No. 369,742, it was found that methyl ethyl ketone (Z-butanone) is a highly etficient solvent in this respect.

Although not as effective as the methyl ethyl ketone solvent, certain solvent mixtures may be used for extracting the organic acids from the spent sulfite liquor, such as disclosed in co-pending application Serial No. 323,144, filed November 28, 1952. As disclosed in this application certain solvent combinations of lower aliphatic esters and lower aliphatic alcohols are also effective in the extraction of organic acids from spent liquor, namely, combinations of solvents having infinite water solubility, such as methyl, ethyl, propyl, isopropyl and tertiary butyl alcohols, with esters having limited solubility, such as, for example, ethyl and isopropyl acetates. Solvents of infinite water solubility such as acetone may also be used in combination with the esters. Specific examples of such solvent mixtures are the following:

In the sulfate cooking operation, soft wood chips are digested under pressure in the conventional manner, using a white cooking liquor whose active ingredients are sodium sulfide and sodium hydroxide. The spent black liquor from this process is removed, by washing, and then concentrated. This liquor contains principally thiolignin, sodium sulfide, sodium hydrogen sulfide, and other extraneous materials.

In accordance with our invention this sulfate process liquor is mixed with the raifinate left from the extraction process of the sulfite spent liquor as described above. The removal of acetic and formic acids from the sulfite spent liquor, prior to mixing with the sulfate liquor, eliminates loss of sulfur as H 5; also, acid corrosion on subsequent handling is prevented.

The resulting mixture is thereupon further concentrated, and then burned in a recovery furnace. The smelt, consisting principally of sodium sulfide and sodium carbonate, is dissolved to produce sulfate green liquor, which in turn is causticized by lime to produce the sulfate white liquor. The white liquor so produced will have a sulfidity as high as 20% to 35% The complete utilization of both the sulfite spent liquor and the sulfate spent liquor in the process virtually eliminates disposal and pollution problems. Also, soda-based sulfite mills that cannot recover their chemicals for reuse can do so through such integration. Salt-cake is no longer required for kraf-t since soda-ash, sodium sulfite, and H completely sustain both mills.

A further economic advantage is that additional heat values may be recovered as steam from the organic matter in the ratfinate from the sodium based sulfite extraction.

The process as described above is applicable in the case of a sulfate process or mill integrated with any sodium based neutral sulfite process.

The following are illustrative but non-limiting examples of the process of our invention.

EXAMPLE I Sodium based neutral sulfite process Approximately 1.33 tons of hardwood chips are digested with a cooking liquor containing 64 pounds of NaHCO expressed as Na O, and 79 pounds Na sO expressed as N330, to produce a ton of pulp and a spent liquor having the following composition:

Pounds sodiumacetate as Na 58.7 Pounds sodium fox-mate as Na O 9.2 Pounds sodium bicarbonate as Na 0 26.7 Pounds NaSO group as Na O 24.3

Pounds total sodium compounds as Na O 118.9

Sulfidity percent 41 The spent liquor is concentrated, acidified with 149.2 pounds of sulfuric acid, the acetic acid (approximately 113.9 pounds) and formic acid (approximately 15.9 pounds) are extracted to leave a rafiinate having the following composition:

Pounds sodium sulfate as Na O 94.6 Pounds NaSO group as Na O 24.3 Pounds total sodium compounds as Na O 118.9 Sulfidity "percent" 120 I he sulfate process Approximately 1.82 tons of soft wood chips are digested in a sulfate white liquor having the following composition:

Pounds Na S as Na O 193.92 Pounds Na SO as Na O 21.55 Pounds NaOH as Na O 463.44 Pounds Na CO as Na O 51.49

Pounds total sodium compounds as Na 0 730.40

Sulfidity percent 29.5

The process will produce one ton of sulfate pulp and a black liquor having the following content:

Pounds total sodium compounds as Na O 657.36 Sulfidity percent 29.5

Mixing the black liquor from the sulfate process with the r-aflinate from the sodium based neutral sulfite process gives a combined black liquor of the following content:

Total sodium compounds as Na o 776.26 Sulfidity percent 43.7

A B O Sulfite Sulfite Spent Salt Cake Raifinate Liquor (100% (120% (41% Sulfidity) Sulfidity) Sulfidity) Lbs. N828 per ton of sulfate pulp. 65. 74 161. 05 193. 92 Lbs. N 32804 per ton of sulfate pulp 7. 30 17. 90 21. 55 Lbs. NaOH per ton of sulfate pulp 591. 62 496. 30 463. 44 Lbs. N21100: per ton of sulfate p p 65.74 55.15 51.49 Lbs. Total alkali per ton of sulfate pulp 730. 40 730. 40 730. 40 Sulfidlty, Percent 10.0 24. 5 29. 5

Expressed as N820.

The following table will show some of the advantages of our invention in terms of chemical recovery and products.

0. Combination of one ton sodium based sulfite with four tons sulfate, salt cake furnishing remaining alkali demanded by sulfate.

D. Complete integration, sulfate and sodium-based sulfite, raflinate supplying chemical demand of the sulfate, after H2504 acidification of sulfite spent liquor and solvent extraction of organic acids and other values, according to the process of our invention.

A B 0 D Pounds total alkali as N320 used per ton of-pul 146 108 117 74 Sulfidity of sulfate white liquor 24. 5 19. 5 29. 5 Pounds acetic acid recovered per ton of pulp 0 0 0 62. 6 Pounds formicacid recovered per ton of pulp 0 0 0 7. 4 Pounds waxes, resins, tannins, etc.,

recovered per ton of pulp 0 0 0 8. 5 Tons pulp per ton alkali as NazO 13. 70 18. 52 17. 09 27. 03 Tons sulfite pulp per ton sulfate 0 0. 25 0. 08

EXAMPLE II The process is carried out as in Example 1, above, except that instead of adding the sulfite raflinate to the concentrated sulfate black liquor, the two are burned separately, the smelts are dissolved and mixed to produce green liquor, and the green liquor causticized to produce sulfate white liquor. The white liquor so produced will have a composition substantially the same as that set forth in Table I, column C, above.

Various modifications and changes may be made in the above procedures, materials and conditions without departing from the scope of the invention as defined in the appended claims.

We claim:

1. A process of integrating neutral sulfite and sulfate pulping operations, in which the amount of neutral sulfite pulp produced is greater than one fourth of the amount of sulfate pulp, comprising producing neutral sulfite pulp, treating the neutral sulfite black liquor therefrom with sulfuric acid, which converts the non sulfur bearing sodium compounds contained in the black liquor, including sodium acetate, sodium formate, and sodium carbonate, into sodium sulfate, acetic acid, and formic acid, removing the acetic acid and the formic acid, leaving a modified neutral sulfite black liquor, producing sulfate pulp, mixing the black liquor therefrom with said modified neutral sulfite black liquor, smelting this mixture and dissolving and causticizing the smelt, said modified neutral sulfite black liquor being capable of satisfying the sodium and sulfur demands of the sulfate pulping operation witl1- out the necessity of adding any other make up chemicals, such as sodium sulfate and sulfur, and whereby the entire amount of neutral sulfite black liquor may be utilized in the integration process.

2. A sulfate wood pulping process which comprises 55 digesting the wood in white liquor to produce pulp and 5 acetic acid and the formic acid have been removed, leaving a modified neutral sulfite black liquor, smelting the mixture of this modified neutral sulfite black liquor and sulfate black liquor, dissolving this smelt to produce green liquor, causticizing this green liquor to produce white liquor, and using this white liquor for digesting the wood.

3. In a sulfate wood pulping process, which comprises the steps of digesting the wood in white liquor toproduce pulp and black liquor, concentrating the black liquor, adding sodium and sulfur components to the black liquor as make up chemical, smelting the resulting mixture, dissolving the smelt to produce green liquor, causticizing the green liquor to produce white liquor, and using the white liquor for digesting the wood, the improvement of supplying the aforesaid sodium and sulfur components in the form of a neutral sulfite black liquor which has been treated with sulfuric acid to convert non sulfur hearing sodium compounds, including sodium acetate, sodium formate, and sodium carbonate, into sodium sulfate, acetic acid, and formic acid, and from which the acetic and formic acids have been removed, the quantity of spent liquor from the sulfite process being suflicient to supply all of the added sulfur for the sulfate process.

4. In an integrated process for pulping wood by a sodium based sulfite process and by the sulfate process, the improvement of producing sulfate white cooking liquor that has sufficiently high sulfidity to avoid the necessity of adding the usual salt cake and sulfur, comprising adding to the sulfate black liquor, concentrated spent liquor from the sulfite process which has been acidified with sulfuric acid and from which substantially all acetic and formic acid have been removed, concentrating the resulting mixture, burning said mixture, dissolving the smelt therefrom to produce sulfate green liquor, and

'causticizing said green liquor, the quantityof spent liquor from the sulfite process being sufficient to supply all of the added sulfur for the sulfate process.

References Cited in the file of this patent UNITED STATES PATENTS 1,017,320 Rinman Feb. 13, 1912 1,298,479 Drewsen Mar. 25, 1919 1,519,508 Bradley Dec. 16, 1924 1,602,553 Richter Oct. 12, 1926 1,637,515 Bradley et al. Aug. 2, 1927 1,795,755 Bradley Mar. 10, 1931 1,833,313 Bradley et al. Nov. 24, 1931 1,834,676 Bradley Dec. 1, 1931 1,859,888 Richter May 24, 1932 1,867,593 Richter July 19, 1932 2,459,570 McGregor Jan. 18, 1949 2,714,118 Copenhaver et a1 July 26, 1955 2,744,927 Copenhaver et al May 8, 1956 

4. IN AN INTEGRATED PROCESS FOR PULPING WOOD BY A SODIUM BOSED SULFITE PROCESS AND BY TE SULFATE PROCESS, THE IMPROVEMENT OF PRODUCING SULFATE WHITE COOKING LIQUOR THAT HAS SUFFICIENTLY HIGH SULFIDITY TO AVOID THE NECESSITY OF ADDING THE USUAL SALT CAKE AND SULFUR, COMPRISING ADDING TO THE SULFATE BLACK LIQUOR, CONCENTRATED SPENT LIQUOR FROM THE SULFITE PROCESS WHICH HAS BEEN ACIDIFIED WITH SULFURIC ACID AND FROM WHICH SUBSTANTIALLY ALL ACETIC AND FORMIC ACID HAVE BEEN REMOVED, CONCENTRATING THE RESULTING MIXTURE, BURNING SAID MIXTURE, DISSOLVING THE SMELT THEREFROM TO PRODUCE SULFATE GREEN LIQUOR, AND CAUSTICIZING SAID GREEN LIQUOR, THE QUANTITY OF SPENT LIQUOR FROM THE SULFITE PROCESS BEING SUFFICIENT TO SUPPLY ALL OF THE ADDED SULFUR FOR THE SULFATE PROCESS. 